Automobile bodies and many industrial and consumer articles are conventionally spray painted in areas called spray booths, wherein water is employed to cleanse the air of over-sprayed paint. The wash water is then treated to remove paint solids, and the treated water is recirculated. The circulating water typically contains less than about 10,000 ppm of suspended solids. A well run system generally contains less than about 500 ppm of suspended solids in the circulating water.
Fine droplets of over-sprayed paint, emitted by a spray gun, contact and are captured by the water. The amount of paint contacting a water curtain may change depending on a number of variables, including plant or process shutdowns, the size and shape of the object being painted, the type of spray equipment used, the spraying and purge technique used, the water flow rate and the type of paint used.
In the past, solvent-based or solvent-borne paints have commonly been employed in spray booths. Federal regulations now limit the amount of volatile organic compounds (i.e., vocs) that can be released at a given plant site. Since organic solvent diluents used in solvent-based paint are a major source of vocs, water-borne or water-based paints are now being used in spray booth operations to help comply with these regulations.
The term "water-based paints", as used herein, refers to all varieties of coatings which contain in excess of approximately 10% water in the coating formulation, including, but not limited to, water-reducible alkyd and epoxy ester compositions, water-borne thermoplastic compositions using acrylic polymer/copolymers, water-based polyurethane dispersions, and blends of such compositions. As used herein, the terms "water-based paints" and "water-borne paints" are synonymous.
A primary treatment objective relative to solvent-based paints concerns the tacky or adhesive nature of the over-sprayed coating material. Due to their hydrophobicity, solvent-based solids tend to coalesce and adhere to the walls, ceilings, floors or spray areas of spray booth systems and in their scrubber sections. Thus, the over-sprayed paint mist captured in the water system of a spray booth must be detackified, or "killed," to prevent accumulation on the walls, piping, etc. of the spray booth system. Paint that sticks to spray booth surfaces usually cannot be easily removed from equipment and tends to build up over time, thereby hampering spray booth efficacy.
In contrast, the primary treatment objective relative to water-based paints is to capture and collect finely-dispersed paint solids. Water-based paints are not tacky in nature. However, without treatment, these paints tend to remain dispersed due to their compatability with water. Ultimately, uncaptured solids accumulate in the system and settle in sludge recovery pits and in booth weirs. Such solids encourage the growth of anaerobic bacteria colonies which may result in odor problems. This treatment problem is aggravated by the use of water-based paints because such paints generally contain resins and dyes which are highly compatible with water.
Other problems which severely interfere with spray booth operations occur in areas of high agitation where foaming occurs and in areas where foam accumulates. Foaming is caused by chemical additives, surfactants, solvents or combinations thereof. Also, finely dispersed paint solids which are not captured and removed tend to stabilize foam, which aggravates foaming problems. Foaming generally mandates that copious amounts of defoamers be used, which results in higher operating costs. Water-based paints generally tend to cause foaming to a greater extent than solvent-based paints.
A wide variety of chemicals have been proposed as treating agents for circulating wet spray booth waters containing over-spray paint, including compositions containing polymers and amphoteric metal salts which form insoluble hydroxides at pH's greater than about 7. The use of combinations of this type are described in the following U.S. Pat. Nos.: 3,861,887 to Forney; 3,990,986 to Gabel et al; 4,002,490 to Michalski et al; 4,130,674 to Roberts et al; and 4,440,647 to Puchalski. Further, U.S. Pat. No. 4,637,824 to Pominville discloses the use of silicates and polydiallyldialkylammonium halides with amphoteric metal salts, and U.S. pat. No. 4,853,132 to Merrell et al discloses the use of precipitates formed by the reaction of cationic polymers and salts of inorganic anions to detackify solvent-based paints. Bentonite clays, aluminum salts and zinc salts have also been used with cationic polymers. U.S. Pat. No. 4,401,574 to Farrington et al discloses the use of polyaluminum chloride to flocculate and settle dispersed paint solids resulting from the production of latex paints and U.S. Pat. No. 4,026,794 to Mauceri discloses water soluble salts of amphoteric metals in combination with dimethyl diallyl ammonium polymers to break oil-in-water emulsions. JP52071538 discloses the use of coagulants such as aluminum sulphate, aluminum polychloride and calcium hydroxide in combination with polymer accelerators to aggregate coating particles in coating booth waste water steams. U.S. Pat. Nos. 4,759,855 and 4,880,471 disclose the use of alkaline zinc solutions containing ammonium hydroxide and ammonium chloride to treat over-sprayed paint.